1. Field of the Invention
The present invention relates to a non-single crystal silicon photovoltaic element having an improved durability and a satisfactory photoelectric conversion efficiency and a module comprising said photovoltaic element. More particularly, the present invention relates a photovoltaic element comprising a non-single crystalline silicon semiconductor material and a back reflector having a light incident side surface with a specific cross section and which excels in durability and exhibits a satisfactory photoelectric conversion efficiency, and a module comprising said photovoltaic element (this module will be hereinafter referred to as photovoltaic element module). The photovoltaic element in the present invention includes a solar cell. The photovoltaic element module in the present invention includes a solar cell module.
2. Related Background Art
For a photovoltaic element (such as a solar cell) having a semiconductor layer comprising a non-single crystalline silicon semiconductor material and having at least a pin junction (this photovoltaic element will be hereinafter referred to as non-single crystal silicon photovoltaic element), there has been proposed a manner of improving the photoelectric conversion efficiency by forming its back reflecting layer with a metal having a high reflectivity against visible light (that is, light having a wavelength in the visible region), such as Ag, Cu, or Al. However, it is known that a metal such as Ag or Cu, which has a high reflectivity against visible light, is liable to cause a so-called migration phenomenon in the presence of moisture and an electric field.
Various studies have been made in order to prevent such migration phenomenon from occurring. For instance, Katoh et al. have reported experimental results of the migration mechanism of Ag series metal (see, R. Katoh and T. Shimizu, Japan Electronic Material Technology Association Bulletin, "anti-migration characteristics of Ag-Pd allow powder"). In addition, in Copper Technology Research Association Bulletin, vol. 30, pages 124-130, "The Characteristics of Electrochemical Migration in Copper-Base Alloy" by T. Tohe et als., there is a disclosure as follows. In order to improve the reliability of electric devices, with respect to the migration resistance of a copper alloy, as a result of examining the kinds of the constituent elements of the alloy and the amount of the elements added, the addition of silicon (Si) provided a significant improvement in the anti-migration characteristics. When nickle (Ni) was added to a Cu--Si alloy, followed by aging, a more improved effect was provided.
Further, when Ag is added with Cu, In, or Pd, or when Cu is added with Ni or Si, it is possible to prevent occurrence of migration to a certain extent. However, in any case, the addition of a different metal is liable to entail a problem of decreasing the reflectivity of the matrix metal (Ag or Cu) against visible light. In this connection, it is considered to be advantageous that the back reflecting layer of a photovoltaic element is comprised of a thin film layer formed of a metallic material containing aluminum as a matrix.
Separately, for a non-single crystal silicon photovoltaic element, there is known a technique wherein a transparent and electrically conductive layer is interposed between a back reflecting layer and a semiconductor layer, and the transparent and electrically conductive layer is designed to have a surface provided with a irregular structure (or a textured structure), thereby providing improved light absorption efficiency. This technique is described, for instance, in Hamakawa et al., Appl. Phys. Lett., 43. P. 644 (1983); T. Tiedje etal., Proc. 16th IEEE Photovoltaic Specialist Conf., P. 1423 (1982); H. Deckman et al., Proc. 16th IEEE Photovoltaic Specialist Conf., P. 1425 (1982); and Japanese Unexamined Patent Publication No. 84888/1986.
For the transparent and electrically conductive layer, it is known that it is advantageous to use zinc oxide in the formation thereof.
Based on the above description, in the case of a non-single crystal silicon photovoltaic element comprising at least a semiconductor layer comprising a non-single crystalline silicon semiconductor material and a back reflecting layer, it is understood to be advantageous that the back reflecting layer is constituted by aluminum, and a transparent and electrically conductive layer constituted by zinc oxide and having a surface with a textured structure is interposed between the back reflecting layer and the semiconductor layer.
However, even in the case of a non-single crystal silicon photovoltaic element having such back reflecting layer and transparent and electrically conductive layer as above described, the non-single crystal silicon photovoltaic element still has a disadvantage such that it is difficult to satisfy the requirements for high photoconductive characteristics, high durability, and low production cost.
Besides this, there are also such disadvantages as will be described hereafter. When forming a zinc oxide thin film on a metal thin film formed of a given metal, the growth of the zinc oxide film is greatly affected by the constituent, surface pattern, and surface cleanliness of the metal thin film. Therefore, it is difficult for the zinc oxide film to be consistently formed so as to have a desirable surface pattern with a textured structure which exhibits a high light collecting efficiency. Hence, it is difficult to consistently form, on a metal thin film composed of a metallic material containing aluminum as a matrix (this metallic material will be hereinafter referred to as "aluminum based metallic material") as the back reflecting layer, a zinc oxide thin film having a desirable surface pattern with a textured structure which exhibits a high light collecting efficiency.
In order to form, on a metal thin film composed of an aluminum based metallic material as the back reflecting layer, a zinc oxide thin film as the transparent and electrically conductive layer so that it has such surface pattern provided with the textured structure, there is known a method of forming said zinc oxide thin film to have a relatively large thickness of about 3 .mu.m, and a method of forming said zinc oxide thin film by means of sputtering using a gas mixture comprising Ar gas and H.sub.2 O gas. However, both of these two methods are problematic. That is, in the case of the former method, particularly the production cost of a photovoltaic element obtained unavoidably becomes costly. In the case of the latter method, there are liable to entail problems such that the deposition rate upon the formation of the zinc oxide film is low, it is difficult for the layered structure comprising the zinc oxide thin film and the aluminum thin film to have a desirably high reflectivity, and it is difficult for a photovoltaic element obtained to have desirably high photovoltaic characteristics, particularly, a desirably high fill factor (F.F.).